Delivery Forces

The uterus itself generates tremendous pressure during contractions. If the average systolic blood pressure (pressure generated by the left ventricle during a contraction) is 120 mmHg, the uterus alone generates more than 5 times that amount. When the accessory muscles are used — the diaphragm and abdominal muscles — the pressure increases to more than 10 times that amount.

The pull generated by a physician during downward traction for shoulder dystocia is slightly higher than systolic pressure and contributes only a tiny amount to the total amount of generated pressures.

Uterine and abdominal pressures are good evidence that if shoulder dystocia is the cause of brachial plexus palsy, it is most likely from the intrinsic pressures of the uterus and body wall, not the caregiver.